Embodiments relate generally to generation of waveforms, and in particular to a synthesized local oscillator and a method of operation thereof.
Broadband synthesizers based on direct digital synthesis (DDS) technology have traditionally led the industry in frequency agility and switching performance, but they have also led in high recurring costs. These high costs stem from the complex nature of DDS-based synthesizers and have resulted in digital synthesizers being utilized primarily in military applications where performance considerations have taken precedence over cost.
Manufacturers of DDS-based synthesizers have long recognized the need to reduce the cost of their products, while still maintaining their lead in performance. Over the years, this has led to the introduction of various modular architectures to reduce complexity. Though these architectures were undoubtedly an improvement, they still did not eliminate the fundamental cost disadvantage of DDS-based synthesizers relative to their analog counterparts.
An existing Direct Digital Synthesizer (DDS) is depicted in FIG. 1. As shown in FIG. 1, the DDS includes a phase accumulator 10 that receives an input corresponding to a change in phase, Δθ. The phase accumulator 10 maintains a running total of the phase. The current phase is output to a phase-to-amplitude converter 12 that generates an amplitude value based on the current phase. The phase-to-amplitude converter 12 generates a representation of a sine wave amplitude based on the phase. This may be performed using a look-up table or other known techniques. Lastly, a digital-to-analog converter (DAC) 14 converts that digital amplitude values to an analog signal to generate a sine wave.
The DDS of FIG. 1 suffers from a high level of noise in the output signal, often referred to as spurious signals or spurs. These spurs may be caused by numerical truncation errors and DAC errors. The spurious signal spectrum can change dramatically with slight changes in the input making filtering difficult.
Different approaches have been implemented to address the spurious signals generated by the basic DDS of FIG. 1. Existing DDS devices incorporate frequency, phase or amplitude dither in an attempt to destroy the coherent nature of the DDS spurious sources. These existing dither techniques produce a high level of output noise, are targeted at reducing circuit complexity rather than improving spurious signals, or are of limited effectiveness in reducing DAC generated spurious signals.
Another approach involves incorporating dither mechanisms to randomize DAC error mechanisms, thus improving output linearity and/or increasing effective resolution. Existing DDS's utilizing dither mechanisms without reducing in-band noise generated directly by the introduced dither signal.
Yet another approach involves filtering the output of the DDS using additional RF hardware to reduce spurious signals. Essentially, a DDS is followed by additional RF circuitry (e.g., dividers, filters) to “clean up” the output spectrum of the DDS. These techniques require a substantial amount of additional RF hardware, and/or limit the modulation capabilities of the DDS.
An existing DDS that addresses drawbacks in conventional DDSs is disclosed in U.S. Pat. No. 6,522,176, the entire contents of which are incorporated herein by reference. While well suited for its intended purposes, it is understood that improvements may be made in controlling the output of the DDS in U.S. Pat. No. 6,522,176 to increase output bandwidth.